Chromic oxide refractory bodies are often used in glass production due to the superior corrosion resistance of chromic oxide.
Generally speaking, glass corrosion resistance of chromic oxide refractory is enhanced by increasing the density and concentration of chromic oxide to eliminate pores which may permit melted glass or slag intrusion and to eliminate other refractory components having less glass corrosion resistance than the chromic oxide. The pores and other components each provide potential sites for corrosion and/or erosion to begin.
Densification of chromic oxide has been obtained by sintering a mixture of chrome sesquioxide (Cr.sub.2 O.sub.3), sometimes also called unreacted or "green" or trivalent chromic oxide, with titania (TiO.sub.2) in a very low or no oxygen content atmosphere. Pure chromic oxide refractories fired without a densifying agent like titania have a maximum bulk density of only about 215 lbs/ft.sup.3. Bulk densities of up to 300 lbs/ft.sup.3 and more have been achieved using titania as a densifying agent.
Chrome sesquioxide particles are formed from clusters of individual crystals which, under microscopic examination, have the general appearance of clusters of grapes. When fired at a sufficiently high temperature, between about 1450.degree. C. and 1600.degree. C., some of the individual chromic oxide crystals grow in size by the absorption of other chromic oxide crystals, while the bulk volume of the material decreases and the bulk density of the material increases.
During densification, titania will enter into solid solution with the chromic oxide as chrome titanate. The amount of titania entering into solid solution is relatively small. A saturated solid solution of chromic oxide and chrome titanate is formed by the combination of chromic oxide with only two to three additional weight percent titania. The densification or increase in bulk density of the chromic oxide also appears to be directly proportional to the amount of titania present (chrome titanate in solution), up to the saturation amount of two to three percent titania or its equivalent in chrome titanate. Excess titania may remain in particle form, be reduced to metallic titanium or possibly combine with other compounds which may be present during sintering.
Generally speaking, densification also can be fostered by the use of titania in combination with other finely divided oxides, particularly silica, to reduce cost by reducing the amount of titania employed.
The term "densified chromic oxide" and like terms are hereinafter used to refer specifically to chromic oxide, the bulk density of which has been increased above about 215 lbs/ft.sup.3 by the inclusion of at least some titania in the green mix before firing or otherwise providing chrome titanate in solution with the chromic oxide during firing.
Densification of chromic oxide has also been reported, at least on a laboratory scale, by the firing of chromic sesquioxide in a bed of carbon in a carbon reducing atmosphere. The formation of chromium carbide by reaction of the chromic oxide and carbon is reported to occur. The specific chemical make-up, thermal shock performance and glass corrosion resistance of this material are unreported. However, carbides are to be avoided in all molten glass contact applications.
The term "sinterable components" is used to refer to metals, metallic oxides, glasses and other materials which remain in a refractory in some form after sintering. These are distinguished from water, volatiles and combustibles which evaporate or are driven out of the composition or consumed (oxidized to a gaseous form) before or during the sintering process.
The term "dense chromic oxide" is used to refer in particular to refractories which are predominantly densified chromic oxide matrix (eighty percent or more by weight Cr.sub.2 O.sub.3) and have a bulk density of at least about 240 and no more than about 255 lbs/ft.sup.3. The term "very dense chromic oxide" is used to refer to refractories which are predominantly densified chromic oxide matrix (about eighty percent or more by weight Cr.sub.2 O.sub.3) and have a bulk density of at least about 255 and no more than about 285 lbs/ft.sup.3. The term "high density chromic oxide" is used to refer to refractories which are predominantly densified chromic oxide matrix (about eighty percent or more by weight Cr.sub.2 O.sub.3) and have a bulk density of at least about 285 lbs/ft.sup.3 or more.
The purification and densification of chromic oxide to increase corrosion resistance typically reduces that material's resistance to thermal shock damage. Thermal shock damage is physical damage such as spalling, cracking and/or fracturing resulting from rapid and/or extreme temperature changes.
Normally, thermal shock damage resistance of dense ceramic bodies can be improved to a certain degree by various means, particularly the use of coarse aggregates. Other means include increased porosity (open or closed), heterogeneous particle densities, and chemistry changes of the base material in the matrix by forming a solid solution of it with another material.
The thermal shock damage resistance of densified chromic oxide has been heretofore improved by the addition of coarse aggregates, namely densified chromic oxide grog. Dense and very dense chromic oxide blocks have been produced this way for use in or in connection with glass furnaces as furnace linings and other glass and slag contact bodies such as flow and bushing blocks. Such chromic oxide refractories are used particularly in the production of textile glass fiber, insulating wool glass fiber, borosilicate glasses and certain other specialty glasses which are considered especially corrosive. This means of enhancing thermal shock resistance in chromic oxide refractories represents a compromise between minimum necessary thermal shock damage resistance and diminished corrosion/erosion resistance.
To reduce the damage from thermal shock in such prior densified chromic oxide refractories used as glass furnace linings, furnace operators have had to carefully control and modify their operating procedures, for example, by providing extremely slow heat-up and cool-down rates, using pressurized heat, etc. It is not uncommon for prior densified chromic oxide refractory blocks forming the lining of a glass melting furnace to crack during the initial heat-up of the furnace, even when such special precautions are taken. Since such furnaces are intended to be in continuous operation for years, even relatively minor thermal shock damage leading to accelerated, localized corrosion/erosion can have a significant impact on the economics of the furnace.
It would be highly valuable to provide chromic oxide refractories having glass corrosion resistance at least comparable to if not greater than those of current densified chromic oxide refractory compositions used in glass furnace applications while providing improved thermal shock resistance.